A typical blowout preventer stack includes a plurality of ram-type preventers surmounted by an annular or bag-type preventer. Typical ram-type preventers are made by Cameron Iron Works, OCT and Schaefer Tool Works. Typical annular preventers are made by Hydril.
In such a stack there are generally at least two ram-type preventers: one with blind rams for closing off communication with the well bore when no pipe extends through the preventers and another with pipe rams for closing off the annulus between the well bore and the pipe. For instance, while the well is being drilled deeper, one of the ram-type preventers have semi-cylindrically concave confronting nose portions capable of being rapidly urged toward one another to circumferentially grip the drill pipe should subterranean pressures suddenly increase to such an extent as to significantly raise the likelihood that the pressure will lift the drill pipe string substantially.
When drilling has reached a depth such that a string of casing is to be run into the well and cemented, it is a wide-spread practice to change out the pipe rams of the respective ram-type preventer and install casing rams in that preventer. The ram-type preventers in widest use are made so that the rams can be exchanged without removing the housing of the preventer from the stack.
Where the driller can afford to tie up more equipment in the drilling of one well, where space permits, or where applicable regulations require it, and where it makes good economic sense, such as in the offshore drilling of wells in deep water with a mudline suspension system, drillers often use a blowout preventer stack which has more than two ram type preventers, for instance one with blind rams, one with rams for drill pipe and others with rams for various diameters of casing to be run.
In former instance, the changing of rams takes time, and each time a set of rams is changed, procedures must be followed to test the integrity of the seals between the peripheries of the rams and the housing of the respective preventer.
It is virtually drilling industry-wide practice to test pipe rams for drilling pipe against a joint of drilling pipe before a stage of drilling is begun, in order to ascertain, before the point of necessity, whether the pipe rams are likely to function properly and hold against a blowout that might happen during the anticipated drilling stage.
This is done by threadably securing a test plug or packer on a joint of drill pipe, lowering this into the well until the test plug or packer is below the blowout preventers, and seating or expanding the test plug or packer into sealing engagement with the well bore. This isolates the preventers from the well for the purpose of testing. Then the pipe rams are closed about the pipe and the annulus outside the pipe, between the test plug and the closed rams is filled with fluid and pressurized. The fluid pressure in the closed volume is monitored over a given period to detect any leakage. A mechanical strain may also be pulled on the pipe at this time to test whether the rams are likely to successfully both contain the pressure and restrain the pipe against upward movement in a blow-out.
The efficacy of the annular preventers is generally similarly tested against the drill pipe. In such a test, the pipe rams are retracted and the toric bag of the annular preventer is radially contracted about the pipe.
It may surprise those not directly connected with well drilling, but it is currently neither the practice nor a specific regulatory requirement that casing rams be similarly tested prior to running a string of casing into a well. (It is a general practice to test the integrity of the seals between the casing rams and the respective blowout preventer body, but that is a matter distinct from testing whether, in the event of a blowout during running and cementing of a string of casing, the casing rams would likely be effective to seal the relevant annulus should the annular preventer also leak, and prevent the casing string from rising).
There are at least two practical reasons why the testing of casing rams has not been routinely practiced.
Prior to the present invention, it would be necessary to run a full joint of casing into the well in order to perform the test. This is impractical.
Test plugs are normally sold with a drill pipe thread on them, because, for years, drillers have run pipe with a test plug in order to test the pipe rams. Drill pipe is typically of relatively small diameter, e.g. 41/2 inches, and has a thread style that is particularly designed to resist twisting off when the drill string is rotated to make a hole. Casing is of varied sizes, a typical well has casing of three different diameters, each being substantially larger in diameter than drill pipe. A typical casing diameter is 103/4 inches.
If drillers were to simply scale the method and apparatus used for testing pipe rams against pipe to the practice of testing casing rams against casing, they would need plugs with casing joint threads, plug they would need to switch to tongs, slips and elevators for handling casing, at a stage when pipe handling tools had been used and would soon need to be used again -- a seeming waste of time, manpower and capital resources.